1. Field of the Invention
The invention relates to polymer compositions which comprise alkoxysilane-terminated polymers, exhibit long skin formation times, and can be activated by adding a catalyst.
2. Description of the Related Art
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. In the presence of atmospheric humidity these alkoxysilane-terminated polymers are able even at room temperature to condense with one another, in the course of which alkoxy groups are eliminated. Depending on the amount of alkoxysilane groups and their construction this condensation is accompanied by the formation primarily of long-chain polymers (thermoplastics), relatively wide-mesh, three-dimensional networks (elastomers) or else highly crosslinked systems (thermosets).
The polymers in question may be alkoxysilane-terminated polymers with an organic backbone, such as polyurethanes, polyesters, polyethers, etc., for example, described inter alia in EP-A-269 819, EP-A-931 800, WO 00/37533, U.S. Pat. No. 3,971,751 and DE-A-198 49 817, or copolymers whose backbone, as well as organic constituents, includes organosiloxanes as well, described inter alia in WO 96/34030.
In accordance with the countless possibilities for designing silane-terminated polymer systems of this kind it is possible to tailor almost arbitrarily the properties of the uncrosslinked polymers and/or of the polymer-containing mixtures (viscosity, melting point, solubilities, etc) and also the properties of the crosslinked compositions (hardness, elasticity, tensile strength, breaking elongation, heat resistance, etc.). A corresponding diversity is therefore also exhibited by the possible uses of such silane-terminated polymer systems. Thus they may be employed, for example, for producing elastomers, sealants, adhesives, elastic adhesive systems, rigid and flexible foams, a wide variety of coating systems, and in the medical sector, such as for impression compounds in the dental sector, for example. These products can be applied in any form, such as by spreading, spraying, pouring, compression molding, etc.
A disadvantage of the majority of alkoxysilane-terminated polymer systems described, however, is their moderate reactivity toward moisture, both in the form of atmospheric humidity and in the form of water—added or already present. In order to obtain a sufficient cure rate even at room temperature, therefore, it is vital to add a catalyst. This is particularly problematic because the organotin compounds generally employed as catalysts are toxicologically objectionable. Moreover, the tin catalysts often still contain traces of highly toxic tributyltin derivatives.
A particular problem is the low reactivity of the alkoxysilane-terminated polymer systems when the desire is for systems which even at room temperature possess a high cure rate—for example, skin formation times <15 minutes. With the usual alkoxysilane-terminated polymers, systems having such reactivity can be obtained, if at all, only with very high concentrations of tin catalysts. This generally results in tin levels of more than 1000 ppm.
The skin formation time here is the period of time which elapses between the application of the polymer blend and the formation of a skin. The skin formation is deemed to have come to an end as soon as the mixture, contacted with a laboratory spatula, for example, no longer remains hanging on the spatula and no longer forms a string when the spatula is removed.
A further decisive disadvantage of the relatively low reactivity of the conventional alkoxysilane-terminated polymer systems lies in the fact that for the majority of applications only methoxysilyl-terminated and not ethoxysilyl-terminated polymers can be used. The ethoxysilyl-terminated polymers specifically would be particularly advantageous, since their curing is accompanied by the release not of methanol but only of ethanol as an elimination product. The ethoxysilyl-terminated polymers, however, possess a reactivity which is further markedly reduced, with the consequence that, using them, it is no longer possible to achieve adequate skin formation rates or cure-through-volume rates at room temperature for the majority of applications.
Distinctly more favorable here are polymer blends which comprise alkoxysilane-terminated polymers having the end group (1) of the general formula-A-CH2—SiR1a(OR2)3-a  (1)where    A is a divalent linking group selected from —O—CO—N(R3)—, —N(R3)—CO—O—, —N(R3)—CO—NH—, —NH—CO—N(R3)— and —N(R3)—CO—N(R3),    R1 is an unsubstituted or halogen-substituted alkyl, cycloalkyl, alkenyl or aryl radical having 1-10 carbon atoms,    R2 is an alkyl radical having 1-6 carbon atoms or an oxaalkyl-alkyl radical having in total 2-10 carbon atoms,    R3 is hydrogen, an unsubstituted or halogen-substituted cyclic, linear or branched C1 to C18 alkyl or alkenyl radical or a C6 to C18 aryl radical, and    a is an integer from 0 to 2.
Silane-terminated polymers of this kind, whose silane termination is separated only by a methylene unit from an adjacent heteroatom, are distinguished by extremely high reactivity toward atmospheric humidity. Thus with methylene-spacered prepolymers of this kind it is possible to prepare blends whose skin formation times even in the absence of any tin catalysts lie in the order of magnitude of just a few minutes. These polymers can include both methoxysilane-terminated polymers and the particularly advantageous ethoxy-silane-terminated polymers.
Compounds corresponding to the general formula (1) have been already described in part, e.g., in DE-A-18 12 562 or DE-A-18 12 564. A disadvantage associated with these polymers, however, is the fact that they were prepared using exclusively aminoalkylsilanes of the general formula (2)NH(R3)—CH2—SiR1a(OR2)3-a  (2)which possess as radicals R3 either hydrogen or else aliphatic groups with little steric bulk, such as cyclohexyl radicals or n-butyl radicals. R1, R2 and a here have the abovementioned definitions.
These compounds are reacted with isocyanate-terminated or else with chloroformic acid-terminated polymers to give the corresponding alkoxysilane-terminated prepolymers.
Although highly reactive isocyanate-free polymer mixtures can be prepared in this way, and can be prepared both from methoxysilyl-terminated polymers and from ethoxysilyl-terminated polymers, these high reactivities neither are controllable nor can be tailored to the particular application.
For instance, in the case of preparation processes corresponding to DE-A-18 12 562 or DE-A-18 12 564, polymers are obtained which, with skin formation times of <<5 minutes possess reactivities which are much too high for many applications and also cannot be reduced or controlled. These polymers are virtually impossible to handle and possess only a low stability on storage. Moreover, such reactive polymers cannot generally be used in customary compounding systems, especially not in filled compositions, since the highly reactive polymers react immediately with the residual moisture present in almost every filler and/or with reactive OH groups on the surface of the filler in question. This generally results in the caking of the composition. Even customary water scavengers, such as vinyltrimethoxysilane, for example, do not allow any improvements to be obtained in this case. Thus the polymer is much more reactive than the conventional water scavenger, and so the latter is unable to fulfill its function of scavenging traces of water prior to a reaction with the silane-terminated polymer.
A further disadvantage is that the addition of a curing catalyst, such as even a common tin catalyst such as dibutyltin dilaurate, for example, cannot be used to adjust the reactivity, since the reactivity of the polymers, which is already extremely high in any case, is virtually impossible to increase. This makes it impossible to use these polymer blends to produce compositions which can be activated only when needed, by addition of a catalyst.
DE-A-21 55 258 and DE-A-21 55 259 propose additionally adding alcohols and acid anhydrides to the polymer blend in order to raise its stability on storage. A disadvantage of this process is, on the one hand, the drastic increase in the amounts of volatile organic compounds, which must evaporate when the alkoxysilane-terminated polymers are cured. For instance, it is necessary to add up to 400% by weight of a solution of alcohols and other organic solvents to the polymers described in DE-A-21 55 258 and DE-A-21 55 259. On the other hand the addition of acid anhydrides leads to acidic compositions, which may attack numerous substrates. As a result, the surface of the substrate is permanently damaged, leading to a substantial loss of adhesion of the respective composition on said surface.
Moreover, these measures only allow a notable improvement to be obtained in the storage stability of the resulting compositions. Thus the systems described in DE-A-21 55 258 and DE-A-21 55 259 continue to possess skin formation times of 1-15 minutes at 25° C. and above 60% atmospheric humidity. Compositions which are slow to react and are stable in air, at least in the short term, and which are activated only by the addition of a suitable catalyst, cannot be obtained by this process.